1. Field of the Invention
The present invention relates to valve actuators, and particularly to power-driven linear actuators for quarter-turn valves such as butterfly, plug, and ball valves.
2. Description of the Prior Art
Many industrial applications for quarter-turn rotary valves such as butterfly, plug, and ball valves call for remote-controlled valve operation, usually with a fail-safe requirement that the valve will shut automatically in the event of failure of the operating power source.
The components of a typical rotary valve actuator for such applications include a linearly reciprocal drive member, an active power source for applying linear force to the drive member, a stored power source for returning the drive member to its initial position, and a linkage for converting the linear motion of the drive member and force of the power source into rotary motion and torque for opening and shutting the valve. The active power source is usually a pneumatic or hydraulic diaphragm motor or piston and cylinder, and a compression coil spring is a typical stored power source.
For a quarter-turn valve, the torque requirements are dictated by three factors: a very high breakaway or opening torque to overcome the friction of the circumferential seal, a minimum torque, constant at all valve positions, due to packing friction, and hydrodynamic torque, which varies with valve position and depends upon the medium being handled and the flow conditions that prevail. For a butterfly valve, the maximum hydrodynamic torque normally occurs at about 70 degrees of valve rotation from the fully shut position.
A typical curve of torque as a function of valve angle for a butterfly valve, for example, will show a high value at the shut position, dropping rapidly to a minimum value of approximately 20% to 35% of the breakaway torque at an opening angle of around 15 degrees. The torque curve then increases to a second maximum value at the above-mentioned 70 degree open position which may be higher or lower than the breakaway torque at opening, depending on the flow rate and flowing medium. At the fully open 90 degree position, the torque curve typically will be lower than that at the 70 degree position but higher than the minimum torque at the 15 degree position. On the other hand, a typical torque curve for a plug valve or a ball valve is also high at opening but drops rapidly and remains low for the remainder of valve travel.
The torque output of the above described reciprocating actuators depends on, among other things, the type of linkage that connects the reciprocating drive member to the valve stem. The most common linkage employed by conventional reciprocating actuators is known as the 45 degree articulated crank. Another popular type of linkage is the scotch yoke.
In the 45 degree articulated crank, a crank arm fastened to the valve stem makes a 45 degree angle with the axis of reciprocation when the valve is shut. For a quarter-turn valve, the crank also makes a 45 degree angle with the drive axis when the valve is fully open. The free end of the crank is connected to the end of the drive member through a link. The drive member in this arrangement is usually offset from the valve stem by a distance such that a line extending along the drive axis intersects the connection point of the link with the crank arm in both the fully open and the shut positions of the valve.
The scotch yoke linkage comprises a crank having a longitudinal slot or the equivalent that engages a roller pin on a reciprocating actuator rod. Usually, the crank of the scotch yoke mechanism also makes a 45 degree angle with the axis of the actuator at the two end positions (i.e., shut and fully open).
Neither the 45 degree articulated crank nor the scotch yoke linkage produces an actuator torque output that reasonably matches the torque curve of a butterfly valve or a plug or gate valve. In particular, the output torque of a 45 degree articulated crank actuator exhibits an upwardly convex curve having a maximum value at about the midpoint and minimums at the shut and fully open positions. The output torque of a scotch yoke actuator, on the other hand, exhibits an upwardly concave curve having a minimum value at approximately the midpoint. Since both the active and the passive power sources must be sized to maintain the actuator torques for opening and shutting the valve, respectively, above the valve torque curve for the full 90 degree operating range, this means that for most of the range the actuator is over-designed. Stated another way, a larger and, therefore, more expensive actuator unit must be fitted for a given application than would be necessary if the maximum actuator output torque positions coincided with the valve maximum torque requirement positions.